time out 8% of the time

I have a wan with 9 satellite clinics each having their own T1 connection to a main site. Each server has nt4.0 with the exception on the one I am having problems with. That one has windows 2000 server…cicso routers, Adtran TSU, and 3 com hubs.The site has been up and running fine for about 6 months then all the sudden network performance dropped at the one particular site. When I ping to or from the main site from here I am getting timed out about6-8% of the time, every time I ping it.I am thinking that this may be on Verizons end but would like suggestions to troubleshoot my end?